Allow SingleSampleMediaSource not to block playback from starting

**Description**

I am using exoplayer 2.11.4 and am adding multiple .srt subtitles to a video using SingleSampleMediaSource and MergingMediaSource.
When the video starts, the player always sends a request for the first subtitles from the list. If delay (timeout) is set on this request (I use Fiddler for this), then the player waits for a response and does not start playing. The player is in BUFFERING state and the user cannot watch the video. I want the subtitle request delay not to interfere with the video playback.

I already ignore 404 subtitle request errors by setting setTreatLoadErrorsAsEndOfStream(true), but that doesn't help in case of a delay.
I also do not allow subtitle re-requests via CaptionsLoadErrorPolicy.

1. Is it possible to turn off the sending of the request for the first subtitles until the user has explicitly selected them?
2. If not, is it possible to make the player allow the video to be played without waiting for the subtitle response?

**My code:**
```kotlin
class CaptionsLoadErrorPolicy : DefaultLoadErrorHandlingPolicy() {
override fun getRetryDelayMsFor(
dataType: Int,
loadDurationMs: Long,
exception: IOException?,
errorCount: Int
) = C.TIME_UNSET
}

const val SUBS_REQUEST_TIMEOUT = 15000

val captionsHttpDataSourceFactory = DefaultHttpDataSourceFactory(
Util.getUserAgent(context, APP_NAME),
SUBS_REQUEST_TIMEOUT,
SUBS_REQUEST_TIMEOUT,
false
)

val format = Format.createTextSampleFormat(
"English",
MimeTypes.APPLICATION_SUBRIP,
Format.NO_VALUE,
"English"
)

val captionsMediaSources = SingleSampleMediaSource.Factory(captionsHttpDataSourceFactory)
.setTreatLoadErrorsAsEndOfStream(true)
.setLoadErrorHandlingPolicy(CaptionsLoadErrorPolicy())
.createMediaSource(uri, format, C.TIME_UNSET)

val videoMediaSource: MediaSource = //video source

val mediaSource = MergingMediaSource(videoMediaSource, captionsMediaSources)

exoPlayer.prepareContent(source = mediaSource, resetPosition = false, resetState = true)
```

**Reproduction steps**
1. Merging video source and multiple .srt subtitles
2. Delay via Fiddler to request the first subtitles (https://some_site/media/subtitle/1f/75/1f75a28e7608df33c786946f39da9b7f.srt)
3. See that the player went to the BUFFERING state, waiting for the subtitle response, during SUBS_REQUEST_TIMEOUT (15 seconds if the response does not come earlier)

**A full bug report captured from the device**
No bug report required as the player does not produce any errors.

**Version of ExoPlayer being used**
ExoPlayer version 2.11.4

**Device(s) and version(s) of Android being used**
Pixel 2 android emulator, Android version 10